Quarks and Gluons in the Nuclear Medium – Opportunities at JLab@12 GeV and an EIC

1. Quarks and Gluons in the Nuclear Medium – Opportunities at JLab@12 GeV and an EIC Rolf Ent, ECT-Trento, June 06, 2008 Nuclear Medium Effects on the Quark and Gluon Structure of Hadrons Main Workshop Topics Nuclear effects in polarized and unpolarized deep inelastic scattering Nuclear generalized parton distributions Hard exclusive and semi-inclusive processes Nuclear hadronization Color transparency Future facilities and experiments

2. The Quark Structure of Nuclei

3. Lattice calculation demonstrates reduction of chiral condensate of QCD vacuum in presence of hadronic matter The QCD Lagrangian and Nuclear “Medium Modifications” The QCD vacuum Long-distance gluonic fluctuations Does the quark structure of a nucleon get modified by the suppressed QCD vacuum fluctuations in a nucleus? Leinweber, Signal et al.

4. Quarks in a Nucleus Observation that structure functions are altered in nuclei stunned much of the HEP community ~25 years ago A=3 EMC Effect at 12 GeV • Effect well measured,over large range of x and A, but remains poorly understood • 1) ln(A) or r dependent? 2) valence quark effect only?

5. gluons valence sea 0.1 1.0 Anti-Quarks in a Nucleus Is the EMC effect a valence quark phenomenon or are sea quarks involved? Tremendous opportunity for experimental improvements! RCa E772 1.0 Deep inelastic electron scattering probes only the sum of quarks and anti-quarks  requires assumptions on the role of sea quarks S. Kumano, “Nuclear Modification of Structure Functions in Lepton Scattering,” hep-ph/0307105 Solution: Detect a final state hadron in addition to scattered electron 0.5 x  Can ‘tag’ the flavor of the struck quark by measuring the hadrons produced: ‘flavor tagging’

6. g1(A) – “Polarized EMC Effect” • New calculations indicate larger effect for polarized structure function than for unpolarized: scalar field modifies lower components of Dirac wave function • Spin-dependent parton distribution functions for nuclei nearly unknown • Can take advantage of modern technology for polarized solid targets to perform systematic studies – Dynamic Nuclear Polarization

7. Valence + Sea Valence only Chiral Quark-Soliton model (quarks in nucleons (soliton) exchange infinite pairs of pions, vector mesons with nuclear medium) Miller, Smith • Valence only calculations consistent with Cloet, Bentz, Thomas calculations • Same model shows small effects due to sea quarks for the unpolarized case (consistent with data) Large enhancement for x>0.3 due to sea quarks Sea is not much modified

8. (polarized EMC effect) g1(A) – “Polarized EMC Effect” • New calculations indicate larger effect for polarized structure function than for unpolarized: scalar field modifies lower components of Dirac wave function • Spin-dependent parton distribution functions for nuclei nearly unknown • Can take advantage of modern technology for polarized solid targets to perform systematic studies – Dynamic Nuclear Polarization Curve follows calculation by W. Bentz, I. Cloet, A. W. Thomas.

9. Extend measurements on nuclei to x > 1: Superfast quarks Fe(e,e’) 5 PAC days Six-quark bag (4.5% of wave function) Mean field Correlated nucleon pair

10. Does the quark structure of a nucleon get modified by the suppressed QCD vacuum fluctuations in a nucleus? Reminder: EMC effect is effect that quark momenta in nuclei are altered • Measure the EMC effect on the mirror nuclei 3H and 3He • Is the EMC effect a valence quark only effect? • Is the spin-dependent EMC effect larger? • Can we reconstruct the EMC effect on 3He and 4He from all measured reaction channels? • Is there any signature for 6-quark clusters? • Can we map the effect vs. transverse momentum/size? Now: use the nuclear arena to look for QCD

11. Use the Nuclear Arena to Study QCD

12. Total Hadron-Nucleus Cross Sections K Hadron– Nucleus total cross section p _ p a p Fit to Hadron momentum 60, 200, 250 GeV/c a = 0.72 – 0.78, for p, p, k a < 1 interpreted as due to the strongly interacting nature of the probe A. S. Carroll et al. Phys. Lett 80B 319 (1979)

13. Physics of Nuclei: Color Transparency Traditional nuclear physics expectation: transparency nearly energy independent. Quantum ChromoDynamics: A(e,e’h), h = hadron 1.0 T Energy (GeV) Ingredients From fundamental considerations (quantum mechanics, relativity, nature of the strong interaction) it is predicted (Brodsky, Mueller) that fast protons scattered from the nucleus will have decreased final state interactions • sh-N cross-section • Glauber multiple • scattering approximation • (or better transport calculation!) • Correlations & Final-State • Interaction effects hN

14. Search for Color Transparency in Quasi-free A(e,e’p) Scattering Fit to s = soAa a Constant value line fits give good description: c2/df = 1 Conventional Nuclear Physics Calculation by Pandharipande et al. (dashed) also gives good description a = constant = 0.75 Close to proton-nucleus total cross section data  No sign of CT yet

15. Physics of Nuclei: Color Transparency Results inconsistent with CT only. But can be explained by including additional mechanisms such as nuclear filtering or charm resonance states. AGS A(p,2p) Glauber calculation The A(e,e’p) measurements will extend up to ~10 GeV/c proton momentum, beyond the peak of the rise in transparency found in the BNL A(p,2p) experiments. Pp(GeV/c) 2.9 5.1 7.3 9.6

16. Physics of Nuclei: Color Transparency A(e,e’p+) 6 7 8 9 10 Total pion-nucleus cross section slowly disappears, or … pion escape probability increases  Color Transparency  Unique possibility to map out at 12 GeV (up to Q2 = 10) Total pion-nucleus cross section slowly disappears, or … pion escape probability increases  Color Transparency?

17. Physics of Nuclei: Color Transparency A(e,e’r+) at 12 GeV (at fixed coherence length) 12 GeV

18. Using the nuclear arena How long can an energetic quark remain deconfined? How long does it take a confined quark to form a hadron? Formation time tfh Hadron is formed { Production time tp Hadron attenuation Quark is deconfined CLAS Time required to produce fully-developed hadron, signaled by CT and/or usual hadronic interactions Time required to produce colorless “pre-hadron”, signaled by medium-stimulated energy loss via gluon emission

19. L p+ e’ pT g* DpT2 = pT2(A) – pT2(2H) “pT Broadening” e Using the nuclear arena How long can an energetic quark remain deconfined? How long does it take a confined quark to form a hadron? Or How do energetic quarks transform into hadrons? How quickly does it happen? What are the mechanisms? dE/dx ~ <pT2>L DE ~ L (QED) ~ L2 (QCD)?

20. p p e’ e Using the nuclear arena How long can an energetic quark remain deconfined? How long does it take a confined quark to form a hadron? Or How do energetic quarks transform into hadrons? How quickly does it happen? What are the mechanisms? Relevance to RHIC and LHC Deep Inelastic Scattering Relativistic Heavy-Ion Collisions Initial quark energy is known Properties of medium are known

21. DpT2 vs. n for Carbon, Iron, and Lead CLAS Hall B Preliminary Pb ~dE/dx ~ 100 MeV/fm (perturbative formula) Fe C DpT2 (GeV2) n (GeV)

22. Production length from JLab/CLAS 5 GeV data (Kopeliovich, Nemchik, Schmidt, hep-ph/0608044) • What we have learned • Quark energy loss can be • estimated • Data appear to support the novel DE ~L2 ‘LPM’ behavior • ~100 MeV/fm for Pb at few GeV, perturbative formula • Deconfined quark lifetime • can be estimated, ~ 5 fm • @ few GeV • Outstanding questions • Higher energy data to • confirm “plateau” for • heavy (large-A) nuclei • Much more theoretical • work needed to provide a • quantitative basis for jet • quenching at RHIC/LHC?

23. Using the nuclear arena DpT2 reaches a “plateau” for sufficiently large quark energy, for each nucleus (L is fixed). DpT2 Projected Data n

24. DOE Project Critical Decisions CD-0 Approve Mission Need CD-1 Approve Alternative Selection and Cost Range Permission to develop a Conceptual Design Report Defines a range of cost, scope, and schedule options CD-2 Approve Performance Baseline Fixes “baseline” for scope, cost, and schedule Now develop design to 100% Begin monthly Earned Value progress reporting to DOE Permission for DOE-NP to request construction funds CD-3 Approve Start of Construction DOE CD3 (IPR/Lehman) review scheduled for July 22-24 DOE Office of Science CD-3 Approval meeting in late Sept 2008 CD-4 Approve Start of Operations or Project Close-out

25. DOE CRITICAL DECISION SCHEDULE Now split in two to ease transition into operations phase Note → 6 to 18 months schedule float included (A) = Actual Approval Date

26. 12 GeV Upgrade: Phases and Schedule (based on funding guidance provided by DOE-NP in June-2007) • 2004-2005 Conceptual Design (CDR) - finished • 2004-2008 Research and Development (R&D) - ongoing • 2006 Advanced Conceptual Design (ACD) - finished • 2006-2009 Project Engineering & Design (PED) - ongoing • 2009-2014 Construction – starts in ~1/2 year! • Parasitic machine shutdown May 2011 through Oct. 2011 • Accelerator shutdown start mid-May 2012 • Accelerator commissioning start mid-May 2013 • 2013-2015 Pre-Ops (beam commissioning) • Hall A commissioning start October 2013 • Hall D commissioning start April 2014 • Halls B and C commissioning start October 2014

27. The Gluon Structure of Nuclei

28. Gluons dominate QCD • QCD is the fundamental theory that describes structure and interactions in nuclear matter. • Without gluons there are no protons, no neutrons, and no atomic nuclei • Facts: • The essential features of QCD (e.g. asymptotic freedom, chiral symmetry breaking, and color confinement) are all driven by the gluons! • Unique aspect of QCD is the self interaction of the gluons • 98% of mass of the visible universe arises from glue • Half of the nucleon momentum is carried by gluons • However, gluons are dark: they do not interact directly with light  high-energy collider!

29. Exposing the high-energy (dark) side of the nuclei • The Low Energy View of Nuclear Matter • nucleus = protons + neutrons • nucleon  quark model • quark model  QCD The High Energy View of Nuclear Matter The visible Universe is generated by quarks, but dominated by the dark glue! Remove factor 20

30. EIC science has evolved from new insights and technical accomplishments over the last decade • ~1996 development of GPDs • ~1999 high-power energy recovery linac technology • ~2000 universal properties of strongly interacting glue • ~2000 emergence of transverse-spin phenomenon • ~2001 world’s first high energy polarized proton collider • ~2003 RHIC sees tantalizing hints of saturation • ~2006 electron cooling for high-energy beams

31. NSAC 2007 Long Range Plan “An Electron-Ion Collider (EIC) with polarized beams has been embraced by the U.S. nuclear science community as embodying the vision for reaching the next QCD frontier. EIC would provide unique capabilities for the study of QCD well beyond those available at existing facilities worldwide and complementary to those planned for the next generation of accelerators in Europe and Asia. In support of this new direction: We recommend the allocation of resources to develop accelerator and detector technology necessary to lay the foundation for a polarized Electron Ion Collider. The EIC would explore the new QCD frontier of strong color fields in nuclei and precisely image the gluons in the proton.”

32. How do we understand the visible matter in our universe in terms of the fundamental quarks and gluons of QCD? Explore the new QCD frontier: strong color fields in nuclei - How do the gluons contribute to the structure of the nucleus? - What are the properties of high density gluon matter? - How do fast quarks or gluons interact as they traverse nuclear matter? Precisely image the sea-quarks and gluons in the nucleon - How do the gluons and sea-quarks contribute to the spin structure of the nucleon? - What is the spatial distribution of the gluons and sea quarks in the nucleon? - How do hadronic final-states form in QCD?

33. Explore the structure of the nucleon • Parton distribution functions • Longitudinal and transverse spin distribution functions • Generalized parton distributions • Transverse momentum distributions

34. Precisely image the sea quarks Spin-Flavor Decomposition of the Light Quark Sea u u u > u d Many models predict Du > 0, Dd < 0 | p = + + + … u u u u d d d d RHIC-Spin region No competition foreseen!

35. GPDs and Transverse Gluon Imaging Deep exclusive measurements in ep/eA with an EIC: diffractive: transverse gluon imaging J/y, ro, g (DVCS) non-diffractive: quark spin/flavor structure p, K, r+, … [ or J/y, f, r0 p, K, r+, … ] Are gluons uniformly distributed in nuclear matter or are there small clumps of glue? Describe correlation of longitudinal momentum and transverse position of quarks/gluons  Transverse quark/gluon imaging of nucleon (“tomography”)

36. GPDs and Transverse Gluon Imaging ,g Fourier transform in momentum transfer x < 0.1 x ~ 0.3 x ~ 0.8 x ~ 0.001 gives transverse size of quark (parton) with longitud. momentum fraction x EIC: 1) x < 0.1: gluons! d x - x x + x 2) x ~ 0  the “take out” and “put back” gluons act coherently. 2) x ~ 0

37. GPDs and Transverse Gluon Imaging Goal: Transverse gluon imaging of nucleon over wide range of x: 0.001 < x < 0.1 Requires: - Q2 ~ 10-20 GeV2 to facilitate interpretation - Wide Q2, W2 (x) range - Sufficient luminosity to do differential measurements in Q2, W2, t EIC enables gluon imaging! Q2 = 10 GeV2 projected data EIC (16 weeks) Scaled from 2 to 16 wks. Simultaneous data at other Q2-values

38. Well mapped in e+p Not so for ℓ+A (nA) Electron Ion Collider (EIC): L(EIC) > 100  L(HERA) eRHIC (e+Au): Ee = 10 (20) GeV EA = 100 GeV seN = 63 (90) GeV LeAu (peak)/n ~ 2.9·1033 cm-2 s-1 ELIC (e+Au): Ee = 9 GeV EA = 90 GeV seN = 57 GeV LeAu (peak)/n ~ 1.6·1035 cm-2 s-1 eA Landscape and a New Electron Ion Collider Terra incognita:small-x, Q  Qs high-x, large Q2

39. F2 : Sea (Anti)Quarks Generated by Glue at Low x F2 will be one of the first measurements at EIC nDS, EKS, FGS: pQCD based models with different amounts of shadowing Syst. studies of F2(A,x,Q2):  G(x,Q2) with precision  distinguish between models

40. FL at EIC: Measuring the Glue Directly Longitudinal Structure Function FL • Experimentally can be determined • directly IF VARIABLE ENERGIES! • Highly sensitive to effects of gluon

41. Explore gluon-dominated matter Longitudinal Structure Function FL • What is the role of gluons and gluon self-interactions in nucleons and nuclei? NSAC-2007 Long-Range Plan Report. • The nucleus as a “gluon amplifier” At high gluon density, gluon recombination should compete with gluon splitting  density saturation. Color glass condensate Oomph factor stands up under scrutiny. Nuclei greatly extend x reach: xEIC = xHERA/18 for 10+100 GeV, Au

42. Diffractive Surprises ‘Standard DIS event’ Diffractive event Detector activity in proton direction No activity in proton direction 7 TeV equivalent electron bombarding the proton … but proton remains intact in 15% of cases … • Predictions for eA for such hard diffractive evens range up to: ~30-40%... given saturation models • Look inside the “Pomeron” • Diffractive structure functions • Diffractive vector meson production ~ [G(x,Q2)]2

43. Explore the transition from partons to hadrons • What governs the transition of quarks and gluons in pions and nucleons? NSAC-2007 • Fragmentation and parton energy loss • The nucleus as a “femto-meter stick” • Nuclear SIDIS: • Suppression of high-pT hadrons analogous but weaker than at RHIC • Clean measurement in ‘cold’ nuclear matter Energy transfer in lab rest frame EIC:10 < n < 2000 GeV (HERMES: 2-25 GeV) EIC: can measure heavy flavor energy loss

44. Using the nuclear arena DpT2 reaches a “plateau” for sufficiently large quark energy, for each nucleus (L is fixed). In the pQCD region, the effect is predicted to disappear (arbitrarily put at n =1000) DpT2 n

45. Quarks and Gluons in the Nuclear Medium – Opportunities at JLab@12 GeV and an EIC Rolf Ent, ECT-Trento, June 06, 2008 Personal View: JLab 12 GeV Upgrade: The 12 GeV Upgrade, with its 1038+ luminosity, is expected to allow for a complete spin and flavor dependence of the valence quark region, both in nucleons and in nuclei. Electron Ion Collider (eRHIC/ELIC) Provide a complete spin and flavor dependence of the nucleon and nuclear sea, study the explicit role that gluons play in the nucleon spin and in nuclei, open the new research territory of “gluon GPDs”, and study the onset of the physics of saturation.

47. FL at EIC: Measuring the Glue Directly Longitudinal Structure Function FL • Experimentally can be determined • directly IF VARIABLE ENERGIES! • Highly sensitive to effects of gluon + EIC alone + 12-GeV data

48. Gluons in the Nucleus eRHIC Note: not all models carefully checked against existing data + some models include saturation physics

49. GPDs and Transverse Gluon Imaging k'  e k q q' * p p' A Major new direction in Nuclear Science aimed at the 3-D mapping of the quark structure of the nucleon. Simplest process: Deep-Virtual Compton Scattering At small x (large W): s ~ G(x,Q2)2 Simultaneous measurements over large range in x, Q2, t at EIC!